[57.01] Mirror Figuring Techniques of Sir William Herschel

E. F. Albin (Fernbank Science Center)

Between the years 1773 to 1818, Sir William Herschel
constructed dozens of speculum telescope mirrors, with
diameters ranging from 6 - 48 inches. Very little, if any,
detailed information has ever been published on the
specifics of his mirror figuring efforts. The reason for
this certainly relates to his desire to closely guard mirror
production trade secrets.

Upon Herschel's death, all telescope-making documents were
passed on to his only son, Sir John Herschel. These
materials are now in the possession of the British RAS and
primarily consist of: a) a four volume series entitled
"Experiments on the Construction of Specula," b) a 129 page
treaty called "On the Construction of Specula," and c) a 179
page manuscript entitled "Results of Experiments on the
Construction of Mirrors." It is suggested that publication
was further delayed and then eventually abandoned due to
silver-coated glass mirrors coming into favor.

A recent investigation by the author, of the unpublished
manuscripts on the construction of specula, suggests that
Herschel's mirror figuring techniques did not involve any
guess work; in fact, his methods were highly refined --
never leaving to chance the evolution of a spherical surface
into the required paraboloid. At the heart of Herschel's
figuring techniques were a series of aperture diaphragms
(similar to the Couder masks used by modern telescope
makers) that were placed over the mirror, which allowed for
the precise determination of its curvature at various
predefined zones. With this information, Herschel was able
to vary his figuring strokes with his polishing tool
accordingly. In addition, all mirrors were subsequently
"star tested," sometimes with aperture diaphragms in place,
allowing for field examination of the mirror's
"distinctness" or performance. Double stars and the planet
Saturn were favorite targets used to analyze and then
correct a mirror's figure.

The author(s) of this abstract have provided an email address
for comments about the abstract:
ed.albin@fernbank.edu